1. Field of the Invention
The present invention relates to a method for producing a ceramic substrate having a surface conductor layer by means of a cofiring process, and to a ceramic substrate produced by the method. More specifically, the present invention relates to a method which is advantageously employed for producing a ceramic substrate that is an assembly of ceramic components and is to be divided into a plurality of ceramic components.
2. Description of the Related Art
In a cofiring process: ceramic green sheets and conductor layers are laminated to thereby form a green ceramic substrate; subsequently, the ceramic bodies and the conductors are cofired to thereby yield a ceramic substrate. In comparison with a postfiring process in which a pattern formed of a conductive paste is printed on a ceramic substrate formed through firing, and then the printed pattern formed of a conductive paste is baked onto the ceramic substrate, the cofiring process is highly advantageous in that surface conductors, such as mounting pads, and a ceramic body can be formed in a collective manner.
However, because the green sheets and the conductor layers shrink at different times in the course of firing, the cofiring process yields a ceramic substrate that is prone to suffer from firing strain, such as warpage. Firing strain is well known to cause defects, such as a defectively mounted condition, in a mounting process.
In order to solve the problem of firing strain, various methods have been proposed. According to a typical known method, a green ceramic substrate is fired while being physically restrained from vertically opposite directions. This allows firing shrinkage to arise predominantly in the thickness direction (Z direction), thereby yielding a ceramic substrate having small firing strain. Specific methods include a method in which a green ceramic substrate is fired while being restrained by means of application of pressure from vertically opposite directions (Patent Document 1 mentioned below) and a method in which a green ceramic substrate is fired while being sandwiched between restraint sheets, which do not shrink in the course of firing, and, after the firing process, the restraint sheets are removed (Patent Document 2 mentioned below).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 62-260777
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 4-243978
3. Problems to be Solved by the Invention
In the method in which a green ceramic substrate is fired while being restrained from vertically opposite directions by use of restraint sheets (Patent Document 2 mentioned above), a restraint sheet is in direct contact with surface conductors, such as mounting pads, formed on the surface of the substrate. Thus, this method possibly involves the following problems: unfavorable surface roughness is transferred onto the surface of the surface conductor layer; foreign matter adheres to the surface of the surface conductor layer; and a plating property and solder wettability of the surface conductor layer are deteriorated. In the method in which a green ceramic substrate is fired while being sandwiched between restraint sheets, which do not shrink in the course of firing, and, after the firing process, the restraint sheets are removed, a problem arises when a thick surface conductor layer is formed (for example, a thickness of 15 μm to 50 μm as measured after firing) in order to impart high bonding strength and solder leaching resistance to the surface conductors. Specifically, surface roughness of the surface conductor layer having an increased thickness causes a restraint sheet to partially fail to be bonded sufficiently to the surface conductor layer. As a result, firing strain is likely to arise in such an insufficiently restrained portion.
The postfiring process may be used to form the surface conductor layer. However, the number of firing operations increases. Also, the postfiring process is unsuited for formation of fine patterns. In the case where the plating process is used to form the surface conductor layer, bonding between the ceramic body and the plating layer becomes insufficient. Also, because a photolithographic process is required, the overall process becomes complicated.